Diboron reagents activation

Important work by Chen et al. [123b] has shown how borylation of alkanes can be achieved both photochemically and thermally from diboron reagents to give alkylboranes (Eq. 2.43). The best catalysts, [CpRh(ethylene)2] and [Cp Rh(r/ -CeMce)], are active at 150°C. The B-B bond oxidatively adds to the metal probably followed by CH oxidative addition. Reductive eMmination gives rise to a new B-C bond being formed. Functionalization occurs at the terminal position of a linear alkane as in the alkane chemistry described above. Since C-B bonds are in principle precursors to a wide variety of functional groups, this reaction has great promise for future development. 

Af-Aiyl amides can be catalytically activated in Ca yi-N or Cat-N cleavage manners depending on catalysts and reaction conditions. Ni(COD)2/IMes catalyses botylation of N-atyl amides with diboron reagents via Cat-N cleavage in the presence of NaOtBu. Ni(COD)2/PCy3 catalyses the reduction of iV-aiyl amides or iV-atyl carbamates with HBpin to afford Ar-H (Scheme 14.37). ... 

Following advances made in reduction reactions (vide supra), hydroboration and diboration have been the subject of intense investigation with NHC-Cu catalysts. Early work by Sadighi revealed that [(ICy)Cu(Ot-Bu)] efficiently catalyzed the 1,2-diboration of aldehydes. Mechanistic studies permitted to rationalize a number of features of this reaction and notably ruled out a possible oxidative addition pathway to favour c-activation of the diboron reagent by the copper centre. [(ICy)Cu(Ot-Bu)] was also used for the diaster-eoselective diboration—in fact, hydroboration after work-up—of sulfinyl aldimines. ... 

Within the last decade, another source of diboron activation has been promoted in basis to the efEcient application to catalysis. This is the use of nanoparticles which can interact with the diboron reagent and develop an enhanced performance toward diboration reaction. Fernandez observed in 2008 that the in situ formation of Au nanoparticles, from Au(I) complexes, could not only activate the B2cat2 but also deliver the boryl units on alkenes with total chemoselectivity. " The gold nanoparticles were estimated to have a mean crystaUite size of 10.5 0.3 nm. The gold nanoparticles were stabilized by 2,2 -bis-(diphenylphosphino)-l-l -binaphthyl (BINAP), diphenylphosphinoethane (DPPE), and L-glutathione. The core size and size distribution ofBINAP-Au nanoparticles were examined by transition electron microscopy (TEM), and the image shows disperse nanopartides 6.9 3.0 nm in diameter (Fig. 6). 

Therefore, this historical prospection toward the activation of diboron has moved from direct reaction of CI2B—BCI2 with alkenes to the current methodology that circumvent the activation of tetra(alkoxy)diboron reagents with simple LBs. But metal activation has been and still is a... 

Sakaki. In fact, the product from the oxidative addition is a saturated hexacoordinated Pd(IV) complex which would require creation of a vacant site in order to enable the coordination of an alkene. The activation ofB2cat2 by [Pd(II)(NHC)Br] resulted more favorably through o-bond metathesis providing [Pd(NHC)(Bcat)] andBrBcat (Scheme 17B) with 3.4 kcal/mol above the reactants (Scheme 17B). Alternatively, the dicationic complex [Pd(II)(NHC)] could form a very stable o-complex with B2cat2, with 32.9 kcal/mol below the two isolated reagents, which promoted the oxidative addition of the diboron (Scheme 17C). 

It seems that all the efforts to activate tetra(alkoxy)diboron compounds are justified since the model CI2B—BCI2 reagent, which develops borylation without metal activation, is too difficult to handle. However, the progress in this field has opened a new window toward the activation of tetra(alkoxy) diboron compounds in a metal-free context by the formation of Lewis add—base adducts and their addition to unsaturated substrates can be performed with total chemo-, regjo-, and stereoselectivity. 

---